南方红土的磁性矿物组成及其区域性差异

位于我国热带、亚热带地区的南方红土是我国分布最广的第四纪土状沉积,其经历了较强的化学风化作用,具有典型的磁性矿物组成。磁性矿物是沉积物的重要组成成分。磁性矿物的种类、粒度和含量等信息可以反映沉积物形成时的温度、降水等气候条件。因此,了解红土沉积物磁性矿物组成对于研究南方红土区的环境演化具有重要意义。本文综合分析了安徽宣城、浙江长兴、江西九江及广西百色4个剖面不同层位红土样品的x-T曲线、三轴等温剩磁热退磁曲线等岩石磁学结果,发现南方红土的磁性矿物组成主要为磁铁矿、磁赤铁矿、赤铁矿及针铁矿,这些磁性矿物的相对含量不仅具有地带性差异,而且在不同风化阶段也存在差异。其中百色剖面顶部棕色土中磁赤铁矿含量相对其他剖面较高,与百色地区现今较强的风化作用相符。同一剖面内网纹红土中赤铁矿的含量较高,磁赤铁矿的含量相对较低,指示网纹化时期较强的成土作用引起磁赤铁矿向赤铁矿的转化。此外,岩石磁学结果显示百色剖面网纹红土及均质红土中含有一种特殊性质的磁性矿物,即粒度较细、解阻温度较低的成土成因赤铁矿,该矿物为后期强烈风化作用的产物。该次生矿物携带了较强的化学剩磁,引起了百色剖面初始碎屑剩磁的重磁化。该研究表明磁性矿物组成的差异还可以用来解释南方红土剩磁记录的多样性。

MAGNETIC MINERALOGY OF THE RED SOIL SEQUENCES IN SOUTHERN CHINA AND ITS VARIETY

Rock magnetic results of four red soil sequences: Xuancheng section in Anhui,Changxing section in Zhejiang,Jiujiang section in Jiangxi and Bose section in Guangxi,have been shown in this paper.The Xuancheng red soil sequence(30°52.406'N,118°51.899'E) has formed on terrace T2 of the Shuiyangjiang River,a tributary of the lower reaches of the Yangtze River.The main pedostratigraphical units of the Xuancheng sequence includes: the Xiashu Loess,red silty clay,vermiculated red silty clay and fluvial conglomerate.Except for the bottom fuvial conglomerate,the Xuancheng sequence is of aeolian origin.The Qiliting red soil sequence(30°54.990'N,119°41.112'E) has formed on terrace T2 of the Xitiaoxi River,a tributary of the Taihu Lake in the lower reaches of the Yangtze River.The Qiliting sequence is mainly comprised of fluvial and alluvial red clay and silts,which is overlain by the Xiashu Loess Formation of aeolian origin and underlain by Cretaceous sandstone.The upper unit of the Changxing sequence is vermiculated red clay and the lower is uniform red clay.Conglomerate clasts sparsely occur in the Changxing sediments.The Jiujiang red soil sequence(29°42'N,116° 00'E) is located near the Jiujiang reporter station of Jiangxi Daily,southwest of Jiujiang City.The Jiujiang sequence is of aeolian origin and can be divided into three lithological units from top to bottom: the loess-paleosol,red clay and vermiculated red clay.The Bose sequence(23°46.664'N,106°43.720'E) has formed on terrace T4 of the Youjiang River in the Bose Basin,Guangxi Province.The Bose sequence is mainly comprised of three lithological units: an upper unit of reddish-brown,a middle unit of vermiculated red clay and a lower unit of red clay.The red soils in subtropical and tropical China are the widest distribution in the form of Quaternary earthy deposits in China.They have experienced a strong chemical weathering,which is correlated with a special magnetic mineral assemblage in red soils.Magnetic minerals are an important component of the sediments.The characteristics of magnetic minerals in the sediments,such as category and relative content,are related to paleoclimatic conditions,like temperature and precipitation.Therefore,the alteration of mineral composition and relative concentration of magnetic minerals plays an important part in understanding the environmental evolution in the red soil region.Rock magnetic results,such as x-T curves and thermal demagnetization of three-component IRM,for samples in different layers of four red soil sequences: Xuancheng,Changxing,Jiujiang and Bose sections, suggest that magnetite,maghemite,hematite and goethite are the main magnetic minerals in red soil sequences in Southern China.However,relative contents of these minerals are various in different regions and different weathering stages. The relative content of maghemite in the upper brown clay of the Bose section is higher than other sections,which is consistent with the stronger weathering in Bose Basin at present.The concentration of hematite is higher and that of maghemite is lower in the vermiculated layer than other layers,which indicates a transformation from maghemite to hematite due to strong pedogenesis when the vermiculated red soils formed. Rock magnetic results also show that a special magnetic mineral,pedogenic hematite was present in the vermiculated and uniform layers of the Bose section.This mineral is a secondary strong weathering product and carries a strong chemical remanent magnetization,which lead to the remagnetization of the primary remanence of the Damei red soil deposits.This study suggests that mineral magnetism can be used to probe into the nature of palaeomagnetic signals of red soil sequences in Southern China.